The Navstar Global Positioning System, hereafter referred to simply as GPS, is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12 hour orbits around the earth.
FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of "on orbit" spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in approximately 12 hours. This positions each of the GPS satellites 101 in such a manner that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
GPS position determination is based upon a concept referred to as time of arrival (TAO) ranging. The orbiting GPS satellites 101 each broadcast spread spectrum microwave signals encoded with positioning data. The signals are broadcast on two frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, with the satellite ephemeris (positioning data in an earth centered, earth fixed, coordinate system) modulated using bi-phase shift keying techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise time of transmission. A user receives the signals with a GPS receiver. The GPS receiver is designed to time the signals and to demodulate the satellite orbital data contained in the signals. Using the orbital data, the GPS receiver determines the time between transmission by the satellite and reception by the receiver. Multiplying this by the speed of light gives what is termed the pseudo range measurement of that satellite. If the GPS receiver clock were perfect, this would be the range measurement for that satellite, but the imperfection of the clock causes them to differ by the time offset between actual time and receiver time. Thus, the measurement is called a pseudo range, rather than a range. However, the time offset is common to the pseudo range measurements of all the satellites. By determining the pseudo ranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT with great accuracy, and use this information to safely and accurately navigate from point to point, among other uses.
Accurate navigation signals and accurate navigation data needs to be available at all times in order to assure safe navigation. For example, it is important to a ship maneuvering through a strait to know precisely its position heading and speed relative to known navigational hazards. Accuracy with respect to the GPS system typically refers to either predictable accuracy (the accuracy of the GPS determined location with respect to the charted location), repeatable accuracy (the ability of GPS receiver to return to the coordinates of a previously determined GPS position), and relative accuracy (the accuracy of a GPS determined position relative to that of another user of the same system at the same time), and each is important safe vessel navigation. Maritime vessels have come to rely upon GPS for accurate navigation data.
The GPS signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is currently specified by DOD to be within 100 meters horizontal positioning accuracy 95% of the time and 300 meters 99.99% of the time. Horizontal accuracy of 100 meters may be adequate for some navigational applications, such as navigating a maritime vessel in open ocean waters, however, maritime navigation in coastal waterways often requires an increased level of accuracy. To provide increased accuracy, the Radio Technical Commission for Maritime Services (RTCM) established standards describing a differential correction GPS service, e.g., messages, format standards, communication bands, and the like.
Differential GPS functions by observing the difference between pseudo range measurements determined from the received GPS signals with the actual range as determined from the known reference station point. The DGPS reference station determines systematic range corrections for all the satellites in view based upon the observed differences. The systematic corrections are subsequently broadcast using the RTCM format to interested users having appropriate DGPS receivers. The corrections enable the users to increase the accuracy of their GPS determined position. Differential service using the RTCM standard is in wide use throughout the world. Tens of thousands of RTCM-compatible receivers have been built and are in operation. These RTCM-compatible receivers, however, are not WAAS-compatible nor are they easily upgraded to WAAS-compatibility.
Referring now to FIG. 2, a schematic diagram of a portion of a differential GPS scheme of operation 200 is shown. Differential GPS scheme 200 shows a plurality of GPS satellites 101, a land based reference station 202, a land based transmitter 203, and a differential GPS receiver equipped user 204. The differential GPS (hereafter DGPS) reference station 202 is fixed at a geodetically surveyed ground position. From this position the reference station tracks the signals of all GPS satellites in view, represented by lines 205. The reference station 202 uses the geodetically surveyed position, and the location of each satellite 101 as determined from the ephemeris data in the satellite transmission, to determine the actual range to the satellite. The reference station then subtracts the measured pseudo range from the actual range to obtain the differential correction for that satellite. After conversion to the RTCM format, the correction is broadcast via transmitter 203 to all users within the local area, e.g., user 204. To insure compatibility, the transmitter 203 broadcasts differential corrections in accordance with an RTCM defined standard. In so doing, any RTCM compliant receiver within range of the transmitter 203 is able to receive and demodulate the differential corrections.
User 204 tracks the GPS satellites in view, represented by lines 206, and determines his "GPS based" position. As described above, this position has a 95% confidence level accuracy on the order of 100 meters. User 204 receives the pseudo range corrections broadcast by the transmitter 203 and applies the corrections to his GPS based pseudo range measurements to derive a DGPS position, wherein the DGPS position has an accuracy of better than 10 meters (typically 1 to 3 meters). In this manner, DGPS greatly improves the accuracy of position information available to user 204. To determine corrections via DGPS methods, user 204 needs to be equipped with a DGPS capable receiver.
Referring now to FIG. 3, a prior art RTCM compatible DGPS receiver system 300 is shown. RTCM compatible DGPS receiver system 300 includes a standard RTCM compatible DGPS receiver 301 adapted to communicate with a RTCM radio demodulator 302 via a standard communications port 303. RTCM compatible DGPS receiver 301 receives GPS satellite signals via antenna 304. The GPS signals are down converted, then de-spread and demodulated by the digital signal processor (DSP) and passed to a processor, which computes the corrected pseudo ranges and determines the GPS based PVT. Information is communicated to the user via a display coupled to the processor and the processor is configured via a coupled user input (e.g., a keyboard or communications port).
RTCM radio demodulator 302 receives RTCM differential correction signals via antenna 305. The RTCM radio demodulator 302 and the RTCM compatible DGPS satellite receiver 301 are both designed and built to comply with the RTCM DGPS standards. Thus, the RTCM differential correction signals are demodulated by the RTCM radio demodulator and passed to RTCM compatible DGPS receiver 301 via a communications port 303. RTCM compatible DGPS receiver 301 applies the DGPS corrections to the GPS pseudo range measurements it determines from the received GPS satellite signals in order to determine a DGPS position. The RTCM radio demodulator can be physically attached to the GPS receiver such that the two form a single physical unit or merely linked to the GPS receiver via communications port 303 and physically mounted at a location away from the RTCM compatible DGPS receiver 301. Additionally, the communications port 303 can be one of many well known interface standards in use in the electronics field (e.g., RS-232, ARINC 429, ARINC 629, Milstd 1553, and the like).
There is a problem however in the fact that in order to determine DGPS positions, the DGPS receiver needs to be within range of an operating DGPS transmitter (hereafter RTCM transmitter). Many RTCM transmitters are located near busy waterways and harbors within the United States. DGPS functions adequately as long as the user remains within range of an RTCM transmitter, often yielding accuracies of one meter or less. In the open ocean, however, and in some waterways and harbors, there is no RTCM transmitter within range (typically 100 kilometers) or of suitable strength to be received. In addition, the farther the DGPS receiver is from the RTCM transmitter, the less accurate the DGPS determined position. In these situations, DGPS position accuracy is reduced. Beyond 100 kilometers, the utility of the corrections deteriorates. This problem has severely limited the utility of RTCM compatible DGPS receivers for the aviation community.
The FAA has recognized GPS overall has great potential benefits for aircraft navigation, and has implemented a new DGPS system called WAAS. Integrating WAAS based DGPS systems with GPS receivers is the subject of this invention. There is an impediment to quick and easy interaction, due to the fact that RTCM compatible DGPS receivers are not capable of easily utilizing wide area augmentation system (WAAS) signals. The FAA plans for WAAS to provide position determination accuracy of 5 meters or less 95% of the time. WAAS, as currently envisioned, will include a network of 22 to 50 ground reference stations with each station determining correction information. The correction information is subsequently processed and uplinked to geostationary satellites for retransmission to users.
The transmitted WAAS signals by themselves are not sufficient to determine a differentially corrected GPS position. An approximate present position is required to properly interpret the transmitted WAAS signals. This approximate present position is typically derived from a conventional, uncorrected GPS present position. Using the approximate present position, a WAAS compatible DGPS receiver uses the WAAS differential correction information to determine an accurate differentially corrected position.
As its name implies, WAAS will be capable of providing DGPS accuracy over very large areas, e.g., the continental United States. Additionally, WAAS provides GPS satellite integrity status by monitoring GPS satellite transmissions in real time and broadcasting integrity or "health" information to WAAS equipped GPS receivers. WAAS signals, however, are not compatible with typical RTCM compatible DGPS receivers. WAAS signals are not RTCM format compliant. As such, RTCM compatible DGPS receivers, e.g., DGPS receiver 300, cannot utilize WAAS capabilities.
Thus, what is required is a method and system of enabling RTCM compatible DGPS receivers to receive differential correction information from the new WAAS signals. Such a system should increase the accuracy of GPS determined positions over wide areas, without regard to distance of a DGPS receiver from a RTCM transmitter. What is further required is a system which allows RTCM compatible DGPS receivers to utilize the GPS satellite integrity monitoring capability of the WAAS network.